1. Field of Invention
The present invention relates to a liquid crystal display panel, a liquid crystal display device and a composite optical element, in particular, to an improved liquid crystal display panel and a liquid crystal display device equipped with the improved liquid crystal display panel as well as to a composite optical element relating to the improvement.
2. Related Art
According to an prior art, a liquid crystal display incorporated in a liquid crystal display (LCD) is illuminated by a surface light source device of side light type. In general, the surface light source device of side light type comprises a guide plate and a primary light source such as rod-like light source which is disposed along an incidence end face of the guide plate. Such arrangement has an advantage such that application to LCD brings only a little increase in thickness.
Primary light emitted from the primary light source is introduced into the guide plate through the incidence end face. According to well known processes including-reflection and scattering, the guide plate deflects the introduced light to produce emission from an emission face provided by a major face. Thus emitted light is supplied to a liquid crystal display panel thorough an additional element such as prism sheet.
Two types of guide plates are known. One type is featured by overall uniform thickness while type is featured by tendency to be decreasing in thickness according to distance from an incidence end face. In general, the latter is capable of emitting illumination light more effectively as compared with the former.
A surface light source device of side light type employing a guide plate of the latter type is shown in FIG. 4 providing an exploded perspective view and in FIG. 5 providing a cross section view along line depicted in FIG. 4. Referring to FIGS. 4 and 5, a surface light source device of side light type 1 comprises a guide plate 2, a primary light source 3, a reflection sheet 4 and a prism sheet 6 as a light control member. The reflection sheet 4, the guide plate 2 and the prism sheet 6 are laminatedly arranged in order. The primary light source 3 is disposed beside an incidence end face 2A of the guide plate 2.
The primary light source 3 consists of, for example, a cold cathode lamp (fluorescent lamp) 8 backed by a reflector 9. Illumination light (primary light) emitted from the fluorescent lamp 8 is supplied to the incidence end face 2A of the guide plate 2 through an opening of the reflector 9. The reflector is a sheet-like member with regular or irregular reflectivity.
The reflection sheet 4 is, for example, a sheet-like member with regular reflectivity to which silver is evaporation-deposited, reflecting light leaked out of the guide plate 2 to return the leaked light into the guide plate 2. This prevents loss of light energy.
The guide plate 2 is a transparent plate-like member made of acrylic resin (PMMA resin) having a wedge-shaped cross section. Such a guide plate 2 is produced, for example, by means of injection molding. Illumination light L introduce into the guide plate 2 through the incidence end face 2A propagates to approach an opposite end face while repeated reflection between a back face 2B and an emission face 2C is effected. Reflection by the back face 2B and that by the emission face 2C involve some emission out of the guide plate 2.
Light emitted from the back face 2B returns to the guide plate 2 directly or via the reflection sheet 4. Light emitted from the emission face 2C is lead to a LCD panel (not shown) via a prism sheet 6.
Light diffusion pattern is applied to the emission face 2C, as required, so that brightness distribution is adjusted by promoting emission from the emission face. This light diffusion pattern consists of, for instance, a great number of rough regions which are distributed according to a certain arrangement rule.
It is well known that the emission face 2C of such a guide plate 2 provides a preferential output which is inclined forward (that is, toward an distal end of wedge-shape). Such property is called “emission directivity ”.
The prism sheet 6 as a light control member is disposed along the emission face 2C of the guide plate 2 to modify such emission directivity. The prism sheet 6 is an optical member an inner face of which provides a prismatic surface, being made of light permeable material such as polycarbonate or polyethylene terephthalate.
The prismatic surface is provided with a great number of projection rows with triangular cross section. Each projection row includes a pair of slopes and runs approximately parallel to the incidence end face 2A.
As well known, emitted light is subject to refraction and inner reflection exerted by slope pairs of the prism sheet 6 arranged as above, with the result that inclined preferential propagation direction (principal propagation direction) is corrected to a frontal direction with respect to the emission face 2C.
Such prior art involves a problem. That is, as the prism sheet 6 is disposed on the emission face 2C of the guide plate 2, static electricity tends to cause the prism sheet 6 to sticK to the emission face 2C.
As a result, prism sheet 6 scrapes against the emission face 2C, thereby causing projection ]rows to be damaged or deformed. In particular, if light scattering pattern is applied to the emission face 2C, this problem will be serious.
Damage or deform of projection rows affects particularly and strikingly top-shapes of projection rows so that tops are eroded off. Such breakage brings white turbidity which is observed from the side of the LCD panel. If the surface light source device 1 is applied to the LCD without elimination such turbidity, display quality of the LCD is reduced without fail.
According an prior art, the prism sheet 6 is made of a soft material in order to overcome this problem. However, such a prism sheet 6 made of a soft material tends to be affected by deformation of cross section after long time operations, thereby deteriorating emission characteristics of illumination light. Beside this, the prism sheet 6 sticks partially to the emission face 2C, with the result that undesirable pattern tends to appear on the emission face 2C.